1. Field of the Invention
This invention relates to radial tires for construction vehicles which largely suppress the crack growth from tread cut failure produced in a belt end to considerably improve the durable life without damaging cut resistance and heat generating durability.
2. Related Art Statement
In general, tires for construction vehicles mainly running on rough roads have a problem that cracks propagate up to a belt end portion due to the increase of strain under an authorized payload also, a heat build-up problem is based on the thickness profile of tire itself and tread cut failure at stone pit and consequently separation failure is caused from the belt end to largely degrade the durable life.
As a technique for solving the above problem, Japanese Patent laid open No. 49-80,703 proposes that two or more different rubber materials having a low heat build-up are arranged side by side in the widthwise direction of the tire so as to match with a deformation system of crown portion and shoulder portion of base rubber during the running for reducing internal heat generation.
Furthermore, Japanese Patent laid open No. 61-287,802 proposes a radial tire having a two layer structure of a tread composed of cap rubber and base rubber, wherein a rubber composition having a good impact cut resistance is used in the center portion of the crown of the base rubber and a rubber composition having a low heat build-up is used in the shoulder portion of the base rubber to thereby differ rubber properties as a tread rubber in the cap rubber and the base rubber at its crown and shoulder portions.
In the technique of Japanese Patent laid open No. 49-80,703, however, the considerable effect of suppressing the heat build-up of the tire is observed by rendering the base rubber into the above moving shape, but since the rubber used is substantially low in heat build-up, the cut resistance is poor so that when cut failure is caused at the tread, there is caused a problem that cracks propagate up to the belt end to finally cause separation failure at belt end.
On the other hand, when the technique of Japanese Patent laid open No. 61-287,802 is applied to the tire for a construction vehicle aiming at the invention, the heat generation level in the central portion of the tread base rubber is high, which comes into problem in view of the heat generating durability, and also since the shoulder portion of the tread base rubber is substantially a low heat build-up rubber, the resistance to crack growth is poor and the cracks are apt to propagate up to the belt end when cut failure is caused in the tread and finally the separation failure at belt end is caused to largely reduce the durable life.
As seen from the above, the aforementioned problems in the tire for construction vehicle can not sufficiently be solved by the above conventional techniques.